Zirconium and hafnium diene and dienyl half-sandwich complexes: synthesis, polymerization catalysis and deactivation pathways. The molecular structures of [M(eta(3)-C3H5)(2,3-Me2C4H4){eta-C5H3(SiMe3)(2)-1,3}](M=Zr or Hf) and [Hf(eta(3)-C3H5){eta(3)-CH2CMeCMeCH2B-(C6F5)(3)}{eta-C5H3(SiMe2)-1,3}]

The reduction of [MCl3Cp ''] [M = Zr or Hf; Cp '' = eta-C5H3(SiMe3)(2)-1,3] with sodium amalgam in the presence of dienes gave the compounds [MCl(diene)Cp ''] which are alkylated with MeMgBr or RMgCl (R = allyl) to give [MX(diene)Cp ''] (X = CH3 or eta(3)-C3H5; diene = 2,3-dimethylbuta-1,3-diene or isoprene). The reduction of [ZrCl(CHCMeCH)Cp ''] with an excess of Na-Hg leads to the binuclear eta(4)(5e)-butadienyl complex [Zr(mu-eta(1):eta(4)-C4H3Me2-2,3)Cp ''](2), also formed from {ZrMe(C4H4Me2)Cp ''] by methane elimination. The butadiene complex [Zr(eta(3)-CH2CMeCHCH2)(eta(2)-C4H6)Cp ''] by methane elimination. The butadiene complex [Zr(eta(3)-CH2CMeCHCH2)(eta(4)-C4H6)Cp ''] is obtained directly from [ZrCl3Cp ''] and MeCHCHCH2MgCl. The complexes [M(allyl)(diene)Cp ''] react with B(C6F5)(3) to give the zwitterionic complexes [Cp '' M+(eta(3)-C3H5)-{eta(3)-(C4H4RRB-)-R-1-B-2(C6F5)(3)}] which contain a 14-electron [CpM(allyl)(2)](+) core stabilised by agostic bonding of the B-CH2 methylene hydrogens. These zwitterions catalyse the polymerisation of ethene to high molecular weight polyethene. Catalysts with similar activities are obtained by the activation of [M(allyl)(diene)Cp ''] with [CPh3]-[B(C6F5)(4)]. The thermal stability of the zwitterionic active species depends strongly on the steric requirements of the dienyl ligands and decreases sharply in the order R-1 = R-2 = Me > R-1 = Me, R-2 = H much greater than R-1 = H, R-2 = H; i.e. the dimethylbutadiene derivatives are stable at room temperature, while in the latter case decomposition is significant even at -60 degrees C. The complexes [Zr(eta(3)-CH2CHCHR1){eta(3)-(CH2CRMeCHCH2B)-Me-2(C6F5)(3)}Cp ''] (R-1 = H, R-2 = Me, 9a; R-1 = Me, R-2 = H, 10) decompose via an unusual C-H activation pathway, with alkene elimination and concomitant migration of a C6F5 substitutuent from boron to zirconium, to give the catalytically inactive borylidene complexes [Zr(C6F5){eta(4)-(CH2CRCHCHB)-C-1(C6F5)(2)}Cp '']. The crystal structures of [M(C3H5)(Me2C4H4)Cp ''] (M = Zr or Hf) and [Hf(eta(3)-CH2CMeCMeCH2B(C6F5)(3)}Cp ''] are reported.